1. Field of the Invention
The present invention relates to an optical recording medium, particularly, to a holographic optical recording medium.
2. Description of the Related Art
A holographic memory that permits recording information in the form of a hologram is capable of recording a large amount of information and, thus, is attracting attention as a recording medium of the next generation. Regarding the photosensitive medium needed for holographic recording, known to the art is a composition which contains, for example, a radically polymerizable monomer, a thermoplastic binder resin, a photo radical polymerization initiator, and a sensitizing pigment as the main components. The photosensitive composition for holographic recording is formed into a film and, then, an interference light exposure is performed so as to achieve the information recording.
In the portion irradiated with light of a high intensity, the polymerization reaction of the radically polymerizable monomer proceeds, and the radically polymerizable monomer is diffused from the portion irradiated with light of a low intensity toward the portion irradiated with light of a high intensity so as to bring about a concentration gradient. In other words, a difference in the density of the radically polymerizable monomer is generated in accordance with the intensity of the interference light so as to bring about a difference in the refractive index.
Proposed in recent years are a recording medium prepared by dispersing a polymerizable monomer in a three dimensionally cross-linked polymer matrix and a recording medium prepared by dispersing an optically polymerizable monomer in an epoxy matrix. In order to form a recording layer, it is necessary for the matrix having a polymerizable monomer dispersed therein to have a reasonable hardness. However, where the matrix is excessively hard, the polymerizable monomer is incapable of migration into the matrix, resulting in failure to obtain a sufficiently large difference in the refractive index. Such being the situation, there was a limit in the recording capacity and in the refractive index modulation of the holographic recording medium. Also, the recording layer tends to be locally shrunk in accordance with the polymerization of the polymerizable monomer. In this case, it is difficult to readout accurately the recorded data.